Two stage electrohydraulic pressure control valves are commonly used in applications where the output pressure is generally proportional to an electronic signal. Such control valves are generally stable when the actuator being controlled by the control valve is subjected to substantially constant pressures or small changes in flow gain during actuation. However, a problem was encountered when those control valves were used to actuate clutches or brakes wherein a piston directly provides engagement force or opposes a spring that provides the engagement force. In both cases, most of the piston movement toward engagement of the clutch or brake occurs at nearly maximum flow at nearly constant pressure. When the clutch or brake actuator piston reaches its end of travel and the brakes or clutches start to engage, the flow demand or gain changes abruptly from high flow with low rate of pressure change to low flow with high rate of pressure change. Since the first stage must respond to the sudden change in flow gain before the second stage can move, the control valve overshoots resulting in harsh or erratic engagement of the clutch or brake.
Thus, it would be beneficial to provide a device to decouple the second stage from the first stage under large changes in flow gain to permit the second stage to quickly adjust to such large changes in flow gain with minimal change in pressure until the first stage can respond for improved stability.
The present invention is directed to overcoming one or more of the problems set forth above.